1. Field of the Invention
This invention relates to acrylonitrile based fibers, oxidized or flame resistant acrylonitrile based fibers obtained by heat-treatment of said acrylonitrile based fibers, carbon fibers obtained by heat-treatment of said oxidized acrylonitrile based fibers, and processes for the production thereof. In accordance with this invention, the time required for oxidation of acrylonitrile based fibers is shortened, and carbon fibers of low fluffing and high strength are obtained.
2. Description of the Prior Art
It has hitherto been known that carbon fibers can be obtained by subjecting acrylonitrile based fibers as the precursor to oxidation treatment (a treatment whereby the fibers are rendered flame resistant) to provide flame resistant fibers and by carbonizing the flame resistant fibers. In this method, however, the time required for the oxidation is quite long, leading to the high production costs of the flame resistant fibers and carbon fibers. Therefore, there has been only a limited demand for these fibers although they have excellent properties.
Furthermore, because of the long oxidation time required, oxidation of a strand of acrylonitrile based fibers is usually carried out in an oxidation furnace provided with a plurality of roller units. In this oxidation treatment, however, some of fibers in the strand are often cut and caused to wind around the rollers. Therefore, operation troubles, formation of strands of high fluffing, and various other problems are likely to take place.
On the other hand, with regard to increasing the strength of carbon fibers, various procedures have been proposed with a certain degree of success, and the strength of the carbon fibers which was initially as low as 200 kg/mm.sup.2 has now been markedly increased. Recently, however, carbon fibers having much higher strengths have been desired.
In order to remove the above described problems, for example, to shorten the time required for the oxidation and to produce strands of higher strength and low fluffing, various investigations have been made. As a result of these investigations, the following findings have been made;
(1) To increase the oxidation rate, copolymerization of acrylonitrile with vinyl monomers containing acidic groups, and incorporation of zinc into acrylonitrile based fibers have been used in the art. It has now been found that when zinc is incorporated into acrylonitrile based polymers consisting of acrylonitrile and vinyl components containing acidic groups, the increase in the oxidation rate of the resulting fibers is greater than separately attained by the copolymerization and the incorporation of zinc. It is also known that the incorporation of zinc into acrylonitrile based fibers results in the production of carbon fibers having increased strength. However, when the content of the zinc in the acrylonitrile based fibers containing acidic groups is more than equimolar the amount of the acidic group, the strength of the carbon fibers obtained from the acrylonitrile based fibers is low. This is considered due to the fact that when a shear force is applied among the molecules constituting the fiber, it is concentrated at the excess zinc.
In general, acrylic fibers are apt to coalescence (stick together) during oxidation and carbonization. The phenomena of coalescence makes adverse effect on strength of the resultant fiber. However, it has been recognized that the fiber specified in the invention can minimize coalescence during oxidation and carbonization.
(2) As the oxidation reaction proceeds, the fiber shrinks. This shrinkage can be divided into a shrinkage caused by the relaxation of the molecular orientation at the beginning of the oxidation reaction, and a shrinkage caused by a cyclization reaction at the late stage of the oxidation reaction. These shrinkages can be distinguished on the basis of the amount of the bonded oxygen.
(3) The introduction of both acidic groups and zinc into acrylontrile based fibers accelerates the oxidation rate. Therefore, the shrinkage at the beginning of the oxidation reaction is greater and more rapid than with those fibers containing no acidic group and no zinc. This is considered due to fixation of the molecular orientation by the components introduced. Therefore, to fix the molecules in the highly oriented state, it is necessary to control the tension applied to the fibers.
(4) The shrinkage created by the cyclization reaction is lower with acrylonitrile based fibers containing zinc and acidic groups than with those fibers containing no zinc and no acidic groups. This is considered due to fixation of the molecular orientation as described above.
(5) Prior art methods required that the fibers be kept in a highly orientated state during oxidation by maintaining them under tension, although the fibers become weaker as the oxidation proceeds. This results in the formation of strands of high fluffing. However, if the molecular orientation is, as described above, fixed at the beginning of the oxidation treatment, it is not necessary to apply high tension. This leads to the reduction in the formation of fluffs.